Apple Academic Press Author Copy...because this water will be wasted, either through deep percolation or surface runoff. 1.3.2 ET c adj WORKSHEET TAB The evapotranspiration is shown

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For Non-Commercial Use

CONTENTS

Abstract ........................................................................................................41.1 Introduction .........................................................................................41.2 Methods ..............................................................................................61.3 Graphical Results ..............................................................................101.4 Discussion .........................................................................................151.5 Summary ...........................................................................................16Acknowledgment .......................................................................................16Keywords ...................................................................................................16References ..................................................................................................16

SIMPLE SPREADSHEET METHOD FOR SCHEDULING IRRIGATION

ERIC W. HARMSEN*

Department of Agricultural and Biosystems Engineering, University of

Puerto Rico, Mayaguez, Puerto Rico 00681, USA

*Corresponding author. *E-mail: [email protected];

[email protected]

CHAPTER 1

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4 Technological Interventions in Management of Irrigated Agriculture

ABSTRACT

A simple spreadsheet method for scheduling irrigation is presented. The water balance method used is based on the methodology of the United Nations Food and Agriculture Organization. Soil moisture is depleted from the soil profile by evapotranspiration. By maintaining the soil mois-ture content between the field capacity and the threshold moisture content, water stress can be avoided and 100% of the potential yield can be achieved, ignoring reductions in yield due to other factors such as fertility, disease, and salinity. The method provides an estimate of the crop stress factor, from which the relative seasonal yield can be estimated for many crops. Several graphs are provided in the spreadsheet, which help the user evaluate their real-time water management. In addition to estimates of the relative yield, the spreadsheet also provides an estimate of the lost irrigation (i.e., irrigation lost to surface runoff or deep percolation). Two examples are provided for the bell peppers located on the southern coast of Puerto Rico.

1.1 INTRODUCTION

Over-application of irrigation can result in loss of water, fuel, and chemicals and may contaminate surface or groundwater. Under-application of water can result in a reduction in crop yield.4 Figure 1.1 shows an example of the relationship between the relative crop yield and the relative seasonal crop water requirement. The curve is based on yield and water-use data for corn, wheat, cotton, barley, pepper, and rice.7–10 Assuming that the regression equa-tion is applicable to crops grown in the Caribbean region and that the loss in net revenue for a crop is directly proportional to the relative yield reduc-tion, Table 1.1 shows the loss, in dollars per acre, for 17 crops grown in the Caribbean region. The net potential revenue can be obtained when 100% of the crop water requirement is applied. The model budget information for the crops listed in Figure 1.2 was obtained from the Conjunto Tecnológico publi-cations of the University of Puerto Rico Agricultural Experiment Station.12 As an example, suppose that the seasonal water requirement for a 10-acre farm of peppers (pimientos) was 400 mm, but only 320 mm was applied and no additional rainfall was received during the season. The relative amount of water applied then was 320 mm/400 mm = 0.8 or 80%. From Table 1.1, the dollar loss for a pepper crop is $129/acre × 10 acres = $1290 for 10 acres (4 ha). The purpose of this exercise is to illustrate the importance of applying the correct amount of water. The values (in dollar) in Table 1.1 are based on

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Simple Spreadsheet Method for Scheduling Irrigation 5

model budgets that are several years old. Therefore, the values in the table should be used for illustration purposes only.

FIGURE 1.1 The relationship between the relative crop yield and the relative seasonal crop water requirement.

FIGURE 1.2 Amount, in dollars, lost per acre as a percentage of crop water requirement applied (*based on model budget data from the Conjunto Tecnológico, UPR Experiment Station12).

Various methods for scheduling irrigation exist, including evapotranspi-ration, soil moisture, and water balance methods. The method discussed in this chapter is a combination of the evapotranspiration and water balance

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methods. The advantage of this method is that soil samples do not need to be taken to obtain the soil moisture content. Harmsen4 presented an evapotrans-piration approach based on daily maps of reference evapotranspiration for Puerto Rico. The operational algorithm provides estimates of daily reference evapotranspiration for these three methods.5,6

This chapter presents simple spreadsheet method for scheduling irrigation.

1.2 METHODS

In this section, a simple spreadsheet method for scheduling irrigation is described. The methodology used in the spreadsheet is based on the United Nations Food and Agriculture Organization (FAO), Irrigation and Drainage Paper 56, Crop evapotranspiration—Guidelines for computing crop water

requirements.1 The spreadsheet can be downloaded from the Internet free of charge from: https://pragwater.com/2011/12/17/a-simple-irrigation-scheduling-spreadsheet-program/. Figures 1.3 and 1.4 show columns A–H

FIGURE 1.3 Columns A–H of the irrigation scheduling spreadsheet (Soil Moisture Worksheet Tab).

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and I–Q of the Soil Moisture Worksheet, respectively, and Table 1.1 lists the definition of these columns. Figure 1.5 shows columns A–H for Application

Rate Worksheet. The user only needs to input data into the yellow highlighted cells. Values in all other cells are calculated automatically. The spreadsheet can be filled-in on a daily basis or could be updated once in a week. It can also be used to evaluate hypothetical scenarios to evaluate the potential

FIGURE 1.4 Columns I–Q of the irrigation scheduling spreadsheet (Soil Moisture Worksheet Tab).

TABLE 1.1 Definitions for the Spreadsheet Columns.

Worksheet Column Column label

Definition

Soil Moisture Worksheet A Date Month/day/yearB FC Field capacity (%)C WP Wilting point (%)D TAW Total available water (%)E RD Root depth (m)F MAD Management allowed deficitG RAW Readily available water (%)H θt Threshold moisture content (%)I θ Moisture content (%)J Ks Crop stress factorK ETc Crop evapotranspiration (mm)

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TABLE 1.1 (Continued)

Worksheet Column Column label

Definition

L ETc adj Adjusted crop evapotranspiration (mm)M Soil water deficit (%)N Irrigation needed (mm)O Applied irrigation or rainfall (mm)P Did stress occur (yes or no)Q Lost irrigation (mm)

Application Rate A Date Month/day/year copied from Soil Mois-ture Worksheet

B Irrigation needed (mm) copied from Soil Moisture Worksheet

C Field area (acre)D Percent wetted area (%)E Irrigation efficiency (%)F Volume of water to apply (gallons)G Pump manifold flow rate (gallons per min)H Time to apply irrigation (h)

FIGURE 1.5 Columns A–H of the irrigation scheduling spreadsheet (Soil Moisture Worksheet Tab).

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crop stress, estimate relative yield, water use, and so on. Owing to space limitations, Figures 1.3–1.5 show only the first 24 days of the crop season.

1.2.1 SPREADSHEET DATA ENTRY

1. In the Soil Moisture Worksheet, enter the date of the first day of the crop season.

2. Enter soil field capacity and wilting point (in %). If user knows the soil texture, one can obtain an estimate of the field capacity and wilting point online from various sources or from textbooks.2

3. The rooting depth needs to be entered for each day of the season. If this information is not available, the user can use the planting depth and the maximum root depth, obtained from the literature, and then creates a linear transition from the planting depth to the maximum depth at the end of the season. The maximum rooting depths for various crops are given in Table 22 of Allen et al.1

4. Management allowed deficit (MAD) is given in FAO 56 Table 221; however, the FAO refers to it as the depletion fraction (P). The P and MAD refer to the same parameters.

5. Enter soil volumetric moisture content (in %) on the first day of the season. After the first day, the soil moisture will be estimated by the spreadsheet.

6. Enter the evapotranspiration (ETc) for each day. Many meteorologic

stations will provide estimates of daily reference evapotranspira-tion (ET

o). User will need to multiply the ET

o by the appropriate

crop coefficient (Kc) for the crop under consideration to obtain ETc.

Growth stage lengths and Kc values for number of crops are available

in Tables 11 and 12, respectively, in FAO Paper 56.1

7. Enter the rainfall or irrigation each day.8. In the next Worksheet Tab (Application Rate), enter the date of the

first day of the crop season.9. In the next Worksheet Tab (Application Rate), enter the area of the

field in acres.10. Enter the percent wetted area. If drip irrigation is used, only a portion

of the field may become wet.11. Enter the percent efficiency of the irrigation system.12. Enter the pump flow rate in gallons per minute.

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1.2.2 USING THE SPREADSHEET TO SCHEDULE THE IRRIGATION

The objective of the analysis is to minimize crop stress as indicated by the crop stress factor (Ks). If this parameter is <1, then the crop is in a state of water stress. To simplify the analysis, the spreadsheet provides the column “Did Stress Occur?” (Column P, Soil Moisture Worksheet). When stress occurs, as indicated by “Yes,” then the user should irrigate the crop. To see how much the user should irrigate, see the column called “Irrigation Needed” (Column N, Soil Moisture Worksheet). Enter the amount the user wants to irrigate in the column called “Applied Irrigation or Rainfall” (Column O, Soil Moisture Worksheet). Finally, go to the Application Rate Worksheet Tab to see how many hours the user should run the irrigation system to put on the desired amount of water.

1.3 GRAPHICAL RESULTS

1.3.1 SOIL MOISTURE GRAPH WORKSHEET TAB

If the soil moisture content falls below the green line (threshold mois-ture content, θt), it indicates that the crop is in stress. Throughout the crop season, the graph should never fall below this line, if possible. It is also very important that the soil moisture does not exceed the field capacity because this water will be wasted, either through deep percolation or surface runoff.

1.3.2 ETc adj WORKSHEET TAB

The evapotranspiration is shown in this worksheet. ETc adj

is the crop ETc

multiplied by the crop stress factor and represents the real ETc. ET

c is also

shown in the graph for comparison.

1.3.3 CROP STRESS FACTOR WORKSHEET TAB

This graph shows the crop stress factor as a function of time for the entire crop season. This line should be as close as possible to 1 throughout the season. Any value lower than 1 indicates that stress occurred. Note that in this worksheet, a value of the relative seasonal crop yield is also provided. The yield response factor (Ky) required to calculate the relative yield can be obtained from Table 24 of Allen et al.1 for various crops.

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1.3.4 CUMULATIVE ET VERSUS IRRIGATION WORKSHEET TAB

The purpose of this graph is to show how close the applied irrigation was to the ET

c. Throughout the season and at the end of the season, the value of the

cumulative irrigation should be as close as possible to the cumulative ETc.

(Note in this graph that the ET used is the ETc and not the ET

c adj).

1.3.5 APPLICATION EXAMPLES

1.3.5.1 EXAMPLE 1

In this example, we will schedule irrigations for a bell peppers. If the crop stress occurred for 1 day, then irrigation was applied the next day at more or less the value of the deficit (“Irrigation needed”). The following information applies to this example:

1. The soil is an silty clay loam having a field capacity and wilting point of 37 and 29%, respectively. The initial soil moisture content is 30%.

2. MAD for peppers is 0.3.3. Initial soil moisture content is 34% for the irrigated area of 5 acre;

the percent wetted area is 100%, irrigation efficiency is 90%; and the irrigation pump is capable of pumping 500 gallons per minute.

4. The location is Puerto Rico’s southern coast.5. The length of the growing season is 85 days (March 14–June 6).6. No rainfall occurred during the growing season.7. The change in rooting depth with time was obtained from the field.8. ET

c was obtained from the equation ETc = Kc × ETo, where, ET

o is

the reference evapotranspiration and the Kc values for peppers were

obtained from the publication Allen et al.1 ETo was obtained from a

weather station. Nowadays, weather stations routinely estimate ETo.

Figure 1.6 shows the soil moisture content during the growth of the bell peppers. Note that irrigation was applied when the soil moisture became close to the threshold moisture content. And the amount of irrigation applied was just enough to increase the soil moisture content to the field capacity. Figure 1.7 shows the ET

c adj and the ET

c. In this case, they are essentially

identical because no stress was allowed to occur (i.e., Ks = 1 during the entire season; Fig. 1.8). Figure 1.9 shows the cumulative ETc versus irrigation.

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FIGURE 1.6 Soil moisture content during the growing season for Example 1.

FIGURE 1.7 ETc adj

and ETc during the growing season for Example 1.

FIGURE 1.8 Crop stress factor (Ks) during the growing season for Example 1.

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FIGURE 1.9 Cumulative ETc and irrigation (plus rain) during the growing season for

Example 1.

The estimated relative yield was 99.3% based on the value of the seasonal response function (Ky) of 1.1 for pepper.1 The total water applied was 464 mm with no loss from surface runoff or deep percolation, and the pump was run for a total of 91.9 h during the season.

1.3.5.2 EXAMPLE 2

In this example, the user will apply 25.4 mm (1 in.) per week, which is a common practice. All other data from Example 1 are the same. Figures 1.10–1.13 show the soil moisture content, the ET

c adj and ET

c, the crop stress

factor, and the cumulative irrigation and ETc during the season.

FIGURE 1.10 Soil moisture content during the growing season for Example 2.

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FIGURE 1.11 ETc adj and ET

c during the growing season for Example 2.

FIGURE 1.12 Crop stress factor (Ks) during the growing season for Example 2.

FIGURE 1.13 Cumulative ETc and irrigation (plus rain) during the growing season for

Example 2.

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The estimated relative yield is 69.5%. Interpolating the dollar loss per acre from Figure 1.2 gives $300.60 per acre or $1503 for 5 acre. The total water applied was 305 mm, with 3.5 mm irrigation lost to surface runoff and/or deep percolation and the pump was run for a total of 60.3 h during the season. This example clearly shows the potential for yield loss for the commonly used “1-inch per week” method.

1.4 DISCUSSION

The spreadsheet method can provide useful real-time information to an irri-gator during the growing season. The method can also be used as an educa-tion tool to better understand the implications of different irrigation scheduling schemes. One of the challenges of using the spreadsheet is the requirement to enter the daily crop evapotranspiration. Operational models of ET

o are

commonly available on the Internet, for example, the website <https://prag-water.com/daily-reference-evapotranspiration-eto-for-puerto-rico-hispanio-laandjamaica/> provides high-resolution daily ET

o for Puerto Rico, the US

Virgin Islands, Dominican Republic, Haiti, Jamaica, and Cuba. The method-ologies, which include ET

o estimates using the Hargreaves–Samani,3 Penman–

Monteith,1 and Priestly–Taylor11 methods, are described in Harmsen et al.5

1.4.1 METHOD LIMITATIONS

Using a spreadsheet method similar to the one described in this chapter is easier than done. If the spreadsheet is not updated frequently, it is quite possible that the crop will begin to experience stress without the grower knowing it. Updating the spreadsheet on a daily basis may not be possible if the source of the ET data is a weather station that needs to be physically downloaded. Weather stations that automatically upload their data to the Internet at least once a day are now commonly available.

Many of the input values used in the spreadsheet may be estimates, for example: field capacity, wilting point, initial soil moisture content (which varies spatially in the field), rooting depth, evapotranspiration, and so on. To the degree that these parameters are in error, then the results from the spreadsheet will also be in error (garbage-in garbage-out).

The spreadsheet graphs may need to be adjusted when the spreadsheet is used with new data. In general, the graphs have been designed with rela-tive horizontal and vertical scales, which change with the data being used.

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However, there may be some data that cause the graphical output to appear incorrectly, in which case the user should make the necessary adjustments to the graphs.

1.5 SUMMARY

Water, fuel, chemicals, and money can be lost if an irrigation system is not managed properly. In this chapter, a simple spreadsheet was presented that can help a farmer to schedule irrigation and achieve good water management. The method is based on the United Nations Food and Agriculture Organiza-tion Paper No. 56. An example problem was presented in which the soil mois-ture was maintained between the field capacity and the threshold moisture content, which resulted in no stress and no yield loss. A second example was presented in which 1 in. of water was applied each week to the pepper crop and the resulting yield and dollar losses were 69.5% and $1503, respectively.

ACKNOWLEDGMENT

Financial support for this study was provided by NOAA CREST under NOAA/EPP grant #NA11SEC4810004 and USDA Hatch Project H-402.

KEYWORDS

• evapotranspiration

• irrigation management

• Penman–Monteith

• soil water deficit

• water stress

REFERENCES

1. Allen, R. G.; Pereira, L. S.; Raes, D.; Smith, M. Crop Evapotranspiration. Guidelines

for Computing Crop Water Requirements; FAO Irrigation and Drainage Paper No. 56, Rome, Italy, 1998, 300 p.

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2. Hargreaves, G. H.; Samani, Z. A. Reference Crop Evapotranspiration from Tempera-ture. Appl. Eng. Agric. ASAE 1985, 1(2), 96–99.

3. Harmsen, E. W. Technical Note: A Simple Web-based Method for Scheduling Irrigation in Puerto Rico. J. Agric. Univ. P.R. 2012, 96, 3–4.

4. Harmsen, E. W.; Mecikalski, J.; Mercado, A.; Tosado Cruz, P. In Estimating Evapo-

transpiration in the Caribbean region Using Satellite Remote Sensing, Proceedings of the AWRA Summer Specialty Conference, Tropical Hydrology and Sustainable Water Resources in a Changing Climate, San Juan, Puerto Rico, August 30–September 1, 2010; pp 1–10.

5. Harmsen, E. W.; Mecikalski, J.; Cardona-Soto M. J.; Rojas Gonzalez, A.; Vasquez, R. Estimating Daily Evapotranspiration in Puerto Rico Using Satellite Remote Sensing. WSEAS Trans. Environ. Dev. 2009, 6(5), 456–465.

6. Irmak, S.; Rathje, W. R. Plant Growth and Yield as Affected by Wet Soil Conditions Due

to Flooding or Over-irrigation; NebGuide, University of Nebraska–Lincoln Extension, Institute of Agriculture and Natural Resources, 2008, 4 p.

7. Jalota, S. K.; Sood, A. J.; Vitale, D.; Srinivasan, R. Simulated Crop Yields Response to Irrigation Water and Economic Analysis: Increasing Irrigated Water Efficiency in Punjab. Agron. J. 2007, 99(July–August), 1073–1084.

8. Ottman, M.; Husman, S. H. Solum Barley; Publication No. 194030. Cooperative Exten-sion, University of Arizona, College of Agriculture, Tucson, Arizona, 1994, 4 p.

9. Oweis, T.; Hachum, A. Water Harvesting and Supplemental Irrigation for Improved Water Productivity of Dry Farming Systems in West Asia and North Africa. Agric.

Water Manag. 2006, 80, 57–73.10. Priestly, C. H. B.; Taylor, R. J. On the Assessment of Surface Heat Flux and Evaporation

Using Large Scale Parameters. Mon. Weath. Rev. 1972, 100, 81–92.11. Fangmeier, D. D.; Elliot, W. J.; Workman, S. R.; Huffman, R. L.; Schwab, G. O. Soil

and Water Conservation Engineering, 5th Edition; Thomson Delmar Learning: Clifton Park, NY, 2006, 552 p.

12. University of Puerto Rico Agricultural Experiment Station, Package Practices (Conjunto

Tecnológico) Model Budget. For Pumpkin, Avocado, Cabbage, Coriander, Cucumber melon, Water melon, Onions, Plantain, Banana, Roots and Tubers. http://www.eea.uprm.edu/publicaciones/conjuntos-tecnologicos/ (accessed January 31, 2016).

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Apple Academic Press Author Copy...because this water will be wasted, either through deep percolation or surface runoff. 1.3.2 ET c adj WORKSHEET TAB The evapotranspiration is shown